The invention relates to a Brillouin optical time domain analyzer device with improved tolerance to degradation and breach of sensing optical fibers.
The field of the invention is, but not limited to, distributed temperature and/or strain sensing using Brillouin scattering.
The use of Brillouin scattering in optical fibers is a well known technique for doing measurements of temperature and/or strain along large distances.
Brillouin scattering occurs when a light wave propagating in a medium (such as an optical fiber) interacts with time-dependent density variations of the medium. These density variations may be due for instance to acoustic waves or phonons propagating in the medium, and they modulate the index of refraction. A fraction of the light wave interacts with these variations of index of refraction and is scattered accordingly. Since acoustic waves propagates at the speed of sound in the medium, deflected light is also subjected to a Doppler shift, so its frequency changes.
The speed of sound in the medium depends on the temperature of the medium and on the strain. So, a variation of any of these parameters induces a variation of the frequency shift of the scattered light due to Brillouin scattering, and so may be measured.
In addition, when an intense beam such as a laser beam travels in a medium such as an optical fiber, the variations in the electric field of the beam itself may produce acoustic vibrations in the medium via electrostriction. The beam may undergo Brillouin scattering from these vibrations, usually in opposite direction to the incoming beam.
Brillouin optical time domain instruments have been done on the basis of this principle. They allow measuring the temperature and/or the strain along distributed sensors based on single-mode optical fibers which may be several kilometers long.
The applications relates mainly to the domains of geosciences, mining, oil exploitation, and civil engineering for the monitoring of large structures.
The distributed sensors are embedded in the environment or the structures to monitor. So, the optical fibers of these distributed sensors are subjected to the variations of temperature and strain of the environment along their path.
The effects of temperature and strain cannot be directly discriminated. So, in order to measure temperature independently of strain, the distributed sensors may comprise optical fibers protected by a small tube or casing which is rigid enough so as to avoid any strain on the fiber along the sensitive part. The length of the fiber in the casing is longer (of an “excess fiber length” EFL) than the casing so that the casing may be stretched in some extends (the EFL) without applying strain to the fiber. The fiber is also usually embedded in a gel in the casing for a better decoupling of strain. So the sensor is sensitive only to temperature and is not affected by strain, provided that strain applied to the distributed sensor remains within some limits (the EFL) so that it does not affect the fiber.
Of course, the distributed sensors may also comprise optical fibers subjected to temperature and strain. The temperature measurements can then be used to compensate for the thermal effects in the unprotected fiber, so as to determine the strain.
Known Brillouin optical time domain instruments are based on one or the other of the two following implementation schemes:                spontaneous Brillouin scattering measurements, in which case the systems are usually referred to as Brillouin Optical Time Domain Reflectors (BOTDR);        stimulated Brillouin scattering measurements, in which case the systems are usually referred to as Brillouin Optical Time Domain Analyzers (BOTDA);        
In spontaneous Brillouin scattering measurements, narrow pulses of light are generated using a continuous wave laser source (usually in the infrared range) and an amplitude modulator or a gating system. These pulses of light are injected into at least one sensing optical fiber of the distributed sensor.
A backscattered optical signal is collected on the same end of this fiber. This optical signal comprises spectral components due to spontaneous Brillouin scattering generated along the sensing fiber by the propagation of the light pulses. These spectral components comprise Stokes and anti-Stokes spectrums located at about ±11 GHz of the central frequency of the laser source, with a spectral width of about 30 MHz. The Stokes spectrum comprises frequency components at frequencies lower than the central frequency of the laser source and the anti-Stokes spectrum comprises frequency components at frequencies lower than that central frequency.
For the detection, the backscattered optical signal is coherently mixed with the laser source wave, used as a local oscillator, on a photodetector. Both waves interfere, which gives rise to an electronic signal with spectral components corresponding to differences of frequencies between the backscattered optical signal and the laser source optical signal. So this electronic signal comprises spectral components around 11 GHz corresponding to the Brillouin scattering. The temperature and/or strain profiles along the fiber may then be obtained from this electronic signal using well-known electronic heterodyne detection methods and/or time-frequency analysis methods.
The spontaneous Brillouin scattering method is quite simple and allows doing instruments of relatively moderate cost because the optical part of the device remains simple. But the sensitivity which may be obtained is low because the spontaneous Brillouin scattering signal is very week. In addition, the photodetector must have a bandwidth larger that the frequency shift of the Brillouin scattering (>12 GHz), which is not favorable to high sensitivity and low noise.
So, in order to obtain accurate and efficient measurements, the stimulated Brillouin scattering method is rather used.
In stimulated Brillouin scattering measurements, narrow pulses of light are also generated using a first laser source (usually in the infrared range). These pulses of light are injected into at least one sensing optical fiber of the distributed sensor.
A continuous probe optical wave is also generated using a second laser source. This second laser source is tunable, as to allow varying the frequency of the probe wave over a frequency range covering the frequency range of the spontaneous Brillouin scattering generated along the sensing fiber by the propagation of the light pulses.
The probe optical wave is injected into at least one second optical fiber of the distributed sensor. The sensing fiber and the second fiber are connected at their distal end so that the probe optical wave travels also in the sensing optical fiber, but in the direction opposite to the light pulses. Of course, the sensing fiber and the second fiber may be just parts of a single or a same optical fiber forming a loop with a forth and back path in the distributed sensor.
When the frequency of the probe optical wave falls within the frequency range of the spontaneous Brillouin scattering generated by the pulsed optical wave, a resonance condition is established, leading to the efficient stimulation of the Brillouin scattering:                when the frequency of the probe optical wave falls within the frequency range of the Stokes spontaneous Brillouin spectrum, this stimulation induces an energy transfer from the pulsed optical wave to the probe optical wave and an amplification of the probe optical wave (gain mode);        when the frequency of the probe optical wave falls within the frequency range of the anti-Stokes spontaneous Brillouin spectrum, this stimulation induces an energy transfer from the probe optical wave to the pulsed optical wave and an attenuation of the probe optical wave (loss mode).        
In these configurations, the resulting optical signal emerging from the sensing optical fiber corresponds essentially to the probe optical wave whose amplitude varies in function of the resonance conditions met along the fiber.
The resulting optical signal may then be detected with a photodetector which electronic bandwidth just needs to be large enough so as to allow obtaining the desired spatial resolution. So a photodetector with an electronic bandwidth narrower than 200 MHz is sufficient for most applications which do not requires shorter than 1 meter spatial resolution.
The frequency of the probe optical wave is varied across the Brillouin frequency range. The analysis of the resulting signals allows an accurate identification of the local resonance condition at every location along the sensing fiber and the computation of the local temperature and strain conditions.
The stimulated Brillouin scattering method allows very accurate measurements.
However, it requires a two-side access to the distributed sensor, or the use of two optical fibers connected at the distal end. So in case of breach of the fibers, or even of one of the fibers, measurements are no longer possible.
This is a serious drawback in practice, because the distributed sensors are usually embedded in structures such as pipelines, with no possibility of replacing them once embedded. In addition they are frequently used in severe environments where the risk to have them subjected locally to excessive strain is high.
It is an object of the invention to provide a device and a method for doing distributed temperature and/or strain measurements with a high sensitivity and a high accuracy.
It is also an object of the invention to provide a device and a method for doing distributed temperature and/or strain measurements which still allows measurements along at least parts of distributed sensors in case of failure or breach of the sensor.
It is a further object of the invention to provide a device and a method which allows exploiting in the best possible conditions healthy and deteriorated distributed sensors.
It is a further object of the invention to provide a device and a method which allows quick and/or automated reconfiguration of the instrumentation to adapt to the health status of the distributed sensors.
It is a further object of the invention to provide a device and a method which allows measurements on various kinds of distributed sensors already installed.